The present invention is in the field of optics and specifically relates to a method for producing an optical pattern in glass.
The concept of treating a glass article to develop a pattern or photographic image therein is old, early references to this concept being provided by U.S. Pat. Nos. 2,326,012 and 2,422,472. These patents disclose a process employing glasses containing certain constituents which impart photosensitivity thereto. Images are formed within the glass by first exposing it to light and thereafter heat-treating the exposed glass to develop a visible image therein. Many subsequent patents have described improvements upon the methods disclosed in the original patents, a recent example being U.S. Pat. No. 4,017,318.
While the above patents have as their object the formation of optical density images in glasses, efforts have also been made to form refractive index patterns therein, that is, patterns formed by localized modifications in the refractive index of the glass. U.S. Pat. No. 4,110,093 and British Pat. No. 1,519,701 describes techniques for forming a refractive index gradient across a glass rod or other article by first impregnating a porous glass preform with a refractive-index-modifying dopant. The dopant is thereafter redistributed by diffusion and finally encased in the glass by a consolidating heat treatment. Unfortunately, refractive index patterns which can be obtained by this method are limited to those resulting from the diffusion process.
Photolyzable organic metallic compounds are known to be useful for the preparation of photosensitive media, and have been considered as an alternatives to precious-metal-containing emulsions. V. I. Gorunov et al describe, in Avtometriya, (1) 71-73 (1978) [CA 89, 97874M], the preparation of light sensitive sheets or films comprising metal carbonyl compounds wherein permanent optical density changes can be induced by exposure and prompt thermal or physical development.
It has also been proposed to deposit metal film patterns on planar substrates such as glass surfaces utilizing organometallic compounds. One method for providing such films is laser-induced chemical vapor deposition, as described by S. D. Allen et al, J. Vac. Sci. Technol., 16(2), (March/April 1979), page 431. An alternative method involves the use of a laser to induce photolytic decomposition of metal alkyls or metal carbonyls, as disclosed by P. M. George et al, Thin Solid Films, 67 (1980), L25-L28, and also by T. F. Deutsch et al, Appl. Phys. Lett., 35(2), July 1979, pages 175-177.
In U.S. Pat. No. 4,403,031, there is described a method for providing an optical pattern in glass wherein a photolyzable organometallic compound is introduced into a porous glass and selectively photolyzed by a suitable source of activating radiation. The exposure is in the form of a pattern or image to be introduced into the glass, and after exposure has been completed, unreacted organometallic compound is typically removed from the pores. As indicated in that patent, a wide variety of transition metal organometallic compounds may be used to develop the desired patterns, and images or patterns in either refractive index or optical density may be provided.
A modification of the process described in U.S. Pat. No. 4,403,031 is described in U.S. Pat. No. 4,488,864. In accordance with the latter patent, after the glass has been impregnated with organometallic compounds and irradiated to produce a pattern in the glass, and unreacted organometallic compound removed therefrom, the porous glass is reimpregnated with a selected organofunctional silicon fluid. Heating of the reimpregnated glass after introduction of the silicon fluid results in polymerization of that fluid in the previously exposed zones, while the fluid outside those zones does not readily polymerize. This results in an intensification of the pattern originally developed in the porous glass.
Polymer impregnation of porous glass is also described by E. A. Chandross et al in "Latent Imaging Photopolymer Systems", Applied Optics, 17 [4] 566-573 (1978). In this process, a polymerization initiator in porous glass is selectively deactivated by a light exposure, and a monomer thereafter introduced into the glass is selectively polymerized by the residual activator. A photopatterned polymer image results, but the refractive index changes attainable are somewhat limited.
As noted in U.S. Pat. No. 4,488,864, the attainment of larger refractive index differentials would be a desirable characteristic of glass devices created by organometallic photolysis. This is true not only for lenses formed in arrays for imaging purposes, but also for devices such as diffraction gratings and planar or cylindrical optical waveguides. The power of lenses or gratings produced by such processes depends upon the product of the index gradient (.DELTA.n) between the treated and untreated portions of the glass, and the thickness (d) of the glass.
A fundamental aim, therefore, has been to both enhance the index change resulting from the process, and also to extend the index modifications more deeply into the glass body in as nearly uniform a manner as possible. Particularly valuable would be a process wherein moderate to large .DELTA.n values could be achieved with relatively brief exposure to the patterning light. With existing processes, the extended exposure of the organometallic compounds to light needed to produce deeper patterns tends to create axial gradients in refractive index along the optical axis of the exposure radiation. This causes distortions and loss of symmetry in the pattern, with such ill effects increasing sharply with exposure time.
In the case of lenses, some advantage can be gained by producing lens arrays in thin cross section, and then stacking arrays to achieve the desired optical path length through the material. However, for applications such as phase gratings, the only practical solution to the problem of low .DELTA.n is to increase the refractive index change resulting from the deposition of metal species in the glass. Most desirably, this would be accomplished in accordance with a process modification wherein the introduction of color centers or absorption sites in the glass can be controlled, so that non-absorbing as well as absorbing patterns could be made.
It is therefore a principal object of the present invention to provide a process for producing optical patterns in glass which results in larger refractive index differentials between index-modified and unmodified regions of the glass.
It is a further object of the invention to provide a process for producing optical patterns in glass which produces moderate-to-large refractive index differentials with only a relatively brief exposure step.
It is a further object of the invention to produce glass products such as phase gratings and planar optical waveguides in durable glasses which exhibit large and/or precisely patterned .DELTA.n values, and thus more efficient refraction or diffraction characteristics.
Other objects and advantages of the invention will become apparent from the following description thereof.